Wednesday, June 12, 2013

Horseshoe crabs (phylum Arthropoda, subphylum Chelicerata, class Merostomata, order Xiphosura, family Limulidae) are the only living members of the class Merostomata.

They can be easily recognised by the horseshoe-shaped shell (or carapace) on the back, and a long, spine-like tail, as shown in the photo above featuring a Coastal Horseshoe Crab (Tachypleus gigas). Contrary to some beliefs, the tail does not give a venomous sting, but is used by the horseshoe crab to right itself if it is overturned, or for steering itself when it swims with its carapace facing downwards (like a boat). They have a pair of compound eyes (one on each side near the front), five simple eyes on top of the carapace, light sensors on its tail, and two more simple eyes on the underside.

Like other arthropods, horseshoe crabs have jointed legs ("arthropoda" means "jointed legs" in ancient Greek), a bilaterally symmetrical body, and an exoskeleton (or external skeleton) composed largely of a tough material called chitin. As they grow, they need to moult, i.e. discard the old skeleton and grow a new one. Unlike many other arthropods, they do not have antennae.

They possess two main body parts - the cephalothorax (head and thorax fused together) and the abdomen, similar to other members of the subphylum Chelicerata. On the underside, there are six pairs of appendages - a pair of chelicerae (specialised appendages used mainly for feeding), a pair of pedipalps (elongate segmented appendages near the mouth for touching and feeding purposes), and four pairs of walking legs. The mouth part is located in the middle. Behind the legs are a set of book gills which comprised a flap-like operculum (which serves a cover for the other gills) and five pairs of leaf-like gills (for breathing and swimming).

During the breeding season, the male will be seen riding on the larger female's back. Fertilisation is external though - the female will lay the eggs, and the male will then deposit the sperm on them.

In intertidal areas with soft substrates, horseshoe crabs can often be seen half buried in the ground, as shown in the photo above featuring a Mangrove Horseshoe Crab (Carcinoscorpius rotundicauda) burrowing into the mud.

Horseshoe crabs are often regarded as living fossils, as studies suggest that their body plan has not changed much for more than 500 million years! This ancient group has an effective system for fighting bacterial infection. There is a special component in their blood which will bind to and inactivate bacteria, fungi and viruses that enter their body. This component becomes gel-like and encloses the intruder inside. Due to this special property, scientists now use horseshoe crab blood to test for bacterial contamination on surgical equipment and drugs. It may sound cruel to be regularly extracting blood from horseshoe crabs for these purposes, but fortunately, scientists from the National University of Singapore have discovered a way to clone the component in the blood that performs this function, and hence fewer horseshoe crabs have to be caught for blood extraction now.

Two species of horseshoe crabs can be seen in Singapore.

The Coastal Horseshoe Crab (Tachypleus gigas) can be recognised by the row of tiny teeth-like spikes at the top of the tail. The cross-section of the tail is triangular.

The tail of the Mangrove Horseshoe Crab (Carcinoscorpius rotundicauda) lacks the tiny spikes and feels somewhat smooth when touched.

While the horseshoe crabs themselves are not poisonous, the roe of the mangrove horseshoe crab is poisonous, and can cause death if consumed.

References

Burnie, D. 2001. Animal. London: Dorling Kindersley. 624 pp.

Ecological Research & Development Group. The Horseshoe Crab. Retrieved May 24, 2013, http://www.horseshoecrab.org